Documentation: kbuild: Add document about reproducible builds

In the Distribution Kernels track at Linux Plumbers Conference there
was some discussion around the difficulty of making kernel builds
reproducible.

This is a solved problem, but the solutions don't appear to be
documented in one place.  This document lists the issues I know about
and the settings needed to ensure reproducibility.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kbuild/index.rst

@@ -18,6 +18,7 @@ Kernel Build System
     headers_install
 
     issues
+    reproducible-builds
 
 .. only::  subproject and html
 

Documentation/kbuild/reproducible-builds.rst

+===================
+Reproducible builds
+===================
+
+It is generally desirable that building the same source code with
+the same set of tools is reproducible, i.e. the output is always
+exactly the same.  This makes it possible to verify that the build
+infrastructure for a binary distribution or embedded system has not
+been subverted.  This can also make it easier to verify that a source
+or tool change does not make any difference to the resulting binaries.
+
+The `Reproducible Builds project`_ has more information about this
+general topic.  This document covers the various reasons why building
+the kernel may be unreproducible, and how to avoid them.
+
+Timestamps
+----------
+
+The kernel embeds a timestamp in two places:
+
+* The version string exposed by ``uname()`` and included in
+  ``/proc/version``
+
+* File timestamps in the embedded initramfs
+
+By default the timestamp is the current time.  This must be overridden
+using the `KBUILD_BUILD_TIMESTAMP`_ variable.  If you are building
+from a git commit, you could use its commit date.
+
+The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
+and enables warnings if they are used.  If you incorporate external
+code that does use these, you must override the timestamp they
+correspond to by setting the `SOURCE_DATE_EPOCH`_ environment
+variable.
+
+User, host
+----------
+
+The kernel embeds the building user and host names in
+``/proc/version``.  These must be overridden using the
+`KBUILD_BUILD_USER and KBUILD_BUILD_HOST`_ variables.  If you are
+building from a git commit, you could use its committer address.
+
+Absolute filenames
+------------------
+
+When the kernel is built out-of-tree, debug information may include
+absolute filenames for the source files.  This must be overridden by
+including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable.
+
+Depending on the compiler used, the ``__FILE__`` macro may also expand
+to an absolute filename in an out-of-tree build.  Kbuild automatically
+uses the ``-fmacro-prefix-map`` option to prevent this, if it is
+supported.
+
+The Reproducible Builds web site has more information about these
+`prefix-map options`_.
+
+Generated files in source packages
+----------------------------------
+
+The build processes for some programs under the ``tools/``
+subdirectory do not completely support out-of-tree builds.  This may
+cause a later source package build using e.g. ``make rpm-pkg`` to
+include generated files.  You should ensure the source tree is
+pristine by running ``make mrproper`` or ``git clean -d -f -x`` before
+building a source package.
+
+Module signing
+--------------
+
+If you enable ``CONFIG_MODULE_SIG_ALL``, the default behaviour is to
+generate a different temporary key for each build, resulting in the
+modules being unreproducible.  However, including a signing key with
+your source would presumably defeat the purpose of signing modules.
+
+One approach to this is to divide up the build process so that the
+unreproducible parts can be treated as sources:
+
+1. Generate a persistent signing key.  Add the certificate for the key
+   to the kernel source.
+
+2. Set the ``CONFIG_SYSTEM_TRUSTED_KEYS`` symbol to include the
+   signing key's certificate, set ``CONFIG_MODULE_SIG_KEY`` to an
+   empty string, and disable ``CONFIG_MODULE_SIG_ALL``.
+   Build the kernel and modules.
+
+3. Create detached signatures for the modules, and publish them as
+   sources.
+
+4. Perform a second build that attaches the module signatures.  It
+   can either rebuild the modules or use the output of step 2.
+
+Structure randomisation
+-----------------------
+
+If you enable ``CONFIG_GCC_PLUGIN_RANDSTRUCT``, you will need to
+pre-generate the random seed in
+``scripts/gcc-plgins/randomize_layout_seed.h`` so the same value
+is used in rebuilds.
+
+Debug info conflicts
+--------------------
+
+This is not a problem of unreproducibility, but of generated files
+being *too* reproducible.
+
+Once you set all the necessary variables for a reproducible build, a
+vDSO's debug information may be identical even for different kernel
+versions.  This can result in file conflicts between debug information
+packages for the different kernel versions.
+
+To avoid this, you can make the vDSO different for different
+kernel versions by including an arbitrary string of "salt" in it.
+This is specified by the Kconfig symbol ``CONFIG_BUILD_SALT``.
+
+.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp
+.. _KBUILD_BUILD_USER and KBUILD_BUILD_HOST: kbuild.html#kbuild-build-user-kbuild-build-host
+.. _KCFLAGS: kbuild.html#kcflags
+.. _prefix-map options: https://reproducible-builds.org/docs/build-path/
+.. _Reproducible Builds project: https://reproducible-builds.org/
+.. _SOURCE_DATE_EPOCH: https://reproducible-builds.org/docs/source-date-epoch/